Here's a rundown of each of the major BB systems currently available, along with advantages, disadvantages & notes on compatibility.

Conventional threaded

Modern threaded bottom brackets are not a new invention.

The idea is simple: by moving the bearings outboard of the shell, you can then use a much larger spindle. Currently, 24mm is the accepted norm compared to square taper's relatively miniscule 17mm diameter.

Shimano popularized the concept as Hollowtech II, with the introduction of the XTR M960 group in 2003 – even now it's still, arguably, the gold standard.

Trek introduced BB90 in 2007, on the revamped Madone, and soon followed with the mountain bike-specific BB95. Instead of having cartridge bearings that were pressed into aluminum cups that were then threaded into the frame, Trek eliminated the middle man and pressed the bare bearings directly into the frame.

Bearing seats were molded into the carbon fiber frame structure, to save weight, and bottom bracket shell width ballooned to 90mm on road bikes and 95mm on mountain bikes, creating more real estate for frame tubes and suspension pivots.

Aside from omitting the cups, the BB90/95 setup is functionally identical to conventional threaded bottom brackets, in that it uses the same bearings and the same bearing locations as before. Crankset compatibility is thus unchanged, although you do lose the ability to run chain guides that sandwich between the driveside cup and frame.

Trek's adherence to the standard it created seems to be wavering, though, as many of the models introduced for 2016 have switched to PF92 (which is covered below).

Pros: Lighter than conventional setups but retains wide range of compatibility; creates a wider BB shell without affecting crank widthCons: Can't use cup-mounted chain guides; shell is wider but still small in diameter; requires a separate bearing puller and press for serviceCrank designs that will fit: Standard 24mm external-type only

PF86/92

Very closely related to Trek's BB90/95 system is the PF86/92 standard used by Scott, Giant, Pivot, and many others. Once again, the bearings and their locations in space are identical to those of conventional threaded bottom brackets, only they're mounted in small composite cups before being pressed into the frame.

The advantages are similar to those for Trek's BB90/95: a wider bottom bracket shell that doesn't otherwise affect crank width, plus lighter weight compared to threaded aluminum cups.

From a manufacturing standpoint, PF86/92's dimensional tolerances aren't as stringent as BB90/95's, and it's easier to adapt the system into aluminum frames. As long as you have a concentric bore of the right diameter, parallel faces, and a proper shell width, you're good to go.

Pros: Same as BB90/95Cons: Same as BB90/95Crank designs that will fit: Standard 24mm external-type, BB386 EVO

BB30

Several companies have dabbled in alternative bottom bracket standards over the years (Gary Fisher, Klein, and Merlin, to name just three) but it wasn't until Cannondale released its then-proprietary BB30 system as an open standard in 2006 that something actually stuck.

BB30's bearing cartridges press directly into a precision-fit shell, as with Trek's BB90/95 system – albeit with the additional aid of two pairs of snaprings. But here it's the shell and spindle diameters that increase in size, not the shell width.

The key benefits here are lighter weights owing to the larger, 30mm-diameter aluminum spindle and the elimination of separate bearing cups. Plus, because the shell is still just 68mm wide, the cranks can also be made to provide more heel clearance – or, depending on the crank design, even a narrower stance width (commonly referred to as 'Q factor').

BB30 shells, however, do require more stringent dimensional tolerances to work well – meaning it's more expensive to manufacture. Moreover, with the notable exception of Felt, who uses a carbon tube, BB30 also usually employs an aluminum shell insert that has to be bonded or co-molded into an otherwise carbon fiber frame.

Cannondale has a variant of BB30 called BB30A. This uses a shell that's 5mm wider than the original but it's an asymmetrical layout that only adds those extra millimeters on the non-driveside. According to Cannondale, this allows for wider bearing spacing and better spindle support (which should improve bearing longevity). Other critical dimensions such as bearing outer diameter and thickness remain unchanged, and there's also no difference in pedal stance width or ankle clearance (in most cases).

Those extra five millimeters can complicate crank compatibility, however. BB30-compatible cranks that feature axle spacers on the non-driveside (such as from Cannondale and SRAM) will work just fine, as all you'll have to do is remove spacers as needed to accommodate the extra width, but BB30 cranks without such spacers (such as from FSA and Specialized) won't work. Thankfully, many newer BB30-compatible cranks now feature extra-long spindles for wider compatibility and those will install just fine.

SRAM addressed BB30's stringent dimensional tolerances and their associated manufacturing costs with the advent of PressFit 30 in 2009. Just as with PF86/92, PF30 is analogous to BB30 in that the bearings and bearing locations are identical, but with the addition of intermediary composite cups that allow for looser frame tolerances.

The Specialized OSBB system

The OSBB is basically the same as BB30 but with a little tighter tolerances and a 0.5mm difference in the clip-to-clip dimension. Cranks of both systems are compatible with each other.

Pros: Same as BB30 but with lower costs, easier assembly, and the potential for additional bearing seals integrated into the cup/bearing assemblyCons: Standard-width shell; requires a separate cup remover and press for serviceCrank designs that will fit: BB30, BB386 EVO, standard 24mm external-type, Specialized OSBB, BBright

However, the non-driveside bearing is pushed outward by 11mm – the same as conventional threaded, BB90, and PF86 bottom brackets. This makes it something of a hybrid between the two widely accepted systems.

Pros: Stiffer than BB/PF30, in theory, without impacting other attributes; good crank compatibility, cheaper to manufacture than threaded or BB30Cons: Questionable longevity; requires a separate cup remover and press for serviceCrank designs that will fit: BBright, BB386 EVO, standard 24mm external-type

BB386 EVO

Takes the oversized principle to extremes. The bearings have the same nominal spacing, width-wise, as conventional threaded, BB90/95, and PF86/92 formats, but it uses BB/PF30's bigger 30mm diameter spindle.

As stiffness is directly tied to cross-sectional size it's no surprise that, theoretically, BB386 EVO has the most to offer, with both the added shell width and diameter that many frame-makers yearn for.

In addition, it uses the same bearings and cups as PF30 and BBright and, thus, the same manufacturing tolerances and required tools.

Pros: Theoretically the best in terms of frame stiffness; reduced manufacturing costs; generous crankset versatilityCons: Heel clearance isn't as good as BB30; requires a separate cup remover and press for serviceCrank designs that will fit: BB386 EVO, standard 24mm external-type

The bottom bracket or 'BB' is the engine room of your bike's chainset and one of the two most important bearings on your machine (the other being the headset).Your BB is located at the junction of your frame's seat- and down-tubes (in a hollow tube referred to as the 'BB shell') and consists of two bearings, one at either side, through which your pedal axle is fitted, enabling the pedals and chainset to spin freely (many cranksets have an integrated pedal axle, three-piece BMX cranksets have a separate one). The BB assembly may also consist of spacers and lockrings, etc. depending on type and design.A good-quality BB, correctly installed, should serve you with years of trouble-free riding, but bearings can wear out or the BB can become loose. This can cause annoying noises or lead to excess movement in the BB ('play') which can damage your frame, so it pays to ensure that your BB is secure and in good condition.When replacing or upgrading your BB there are a number of things to take into account, with models varying according to bike type (BMX BBs are different from those used on road and MTB bikes), BB shell size (they can be different lengths so you need a BB to fit) and sometimes, frame design (many modern road and MTB bikes use 'press-fit' designs where the shell is moulded to enable the bearings fit directly in).

MTB and road bottom brackets are functionally the same as they ever were, consisting of two sets of bearings which allow the pedals and crankset to rotate freely.However in terms of design they have evolved considerably in recent years, with a trend towards weight-shedding minimalism which means that many 'bottom brackets' are now little more than two bearings. A further trend has been towards integration of components, which has seen the pedal axle evolve to become an integral part of the crankset, and the BB's bearing cups incorporated into the frame itself in some instances.

To some extent, the type of bottom bracket you choose will be determined by your frame type, as some frames – particularly carbon models above a certain price point – are made to use one type of BB, and one type only.With some BB types (anyone remember cottered cranks?) having died out completely, there are three main designs remaining – the cartridge-type BB (itself a declining standard); the external or outboard BB and the press-fit BB (with this latter type becoming more and more commonplace).Inexpensive, easy-to-fit and well-sealed cartridge BBs are still found on lots of MTB and road frames at the more budget or utilitarian end of the market, with external BBs at one point having largely replaced them on higher-end models. However the increased use of carbon fibre as a frame material has also led to a growth in press-fit BBs being specced as standard on mid- to high-end carbon bikes, as well as some aluminium models. It's one area we are seeing a lot of divergence in at the moment, with different manufacturers using different standards, for different claimed benefits.Whichever type you choose – or are obliged to stick with - make sure you choose the correct width to fit your BB shell. These are generally 68 or 73mm wide on MTBs, or 68/70mm on road bikes although again this is not such a consideration with press-fit BBs.

The Most Common Types of Bottom Brackets: -

• Cartridge BB: This older standard features a small diameter axle and internal bearings sealed in a metal cylinder which screws into the BB shell as a single unit. Whereas the predecessors of this design required painstaking bearing adjustment using two spanners, cartridge BBs became popular as a 'fit and forget' solution. Once they wore out, you simply replaced them, rather than fiddling around with bearings.Again, this standard is in decline but can still be found on older and entry-level MTB and road bikes. When replacing, check that your axle type matches your cranks. Pedal axles for cartridge BBs may be square-taper or splined at the interface with the cranks, while some chainsets use integrated axles that are not designed for use with cartridge BBs. Most cartridge BBs will have their model no. and specifications printed on an external sticker, making them straightforward to replace. While cartridge BBs are heavier than their external or press-fit cousins and not as stiff, lots of people still like them – they are super-easy to fit at home, relatively inexpensive and have a good reputation for keeping out water and lasting for years.

• External BB or 'conventional threaded': This is where cartridge bearings sit in aluminium cups on the exterior of the BB shell ('outboard' bearings). The principle behind this is that it allows for the use of a larger-diameter axle (typically 24mm as opposed to the 17mm used in cartridge BB axles) for increased stiffness in the BB area and therefore improved power transfer.The external BB setup means greater stiffness for less weight, but it can have drawbacks. Bearings can die quickly if they are not perfectly aligned, and it is often advised to have your BB shell professionally faced (where the opposite sides are ground down to ensure they are perfectly parallel) for maximum longevity. Ceramic bearings are also regarded as longer-lasting and smoother-running than standard steel ones.

• Press-fit: Press-fit bottom brackets are found on an increasing number of mid- to high-end carbon fibre MTB and road bikes, and some aluminium models. With these, the bearings are pressed directly into the frame, either in aluminium cups or into races that are moulded into the structure of the frame. The main advantage of the press-fit generation of BB standards is that is allows for a wider BB shells without affecting axle width, thus enabling bigger tube sizes to be used and increasing frame stiffness in the BB area. This, in turn, improves the power transfer between rider and frame by reducing flex at the BB. However there are many different sizes and iterations according to manufacturer – BB30, PF30, BB90 ,PF86/92 to name but a few – and not all are interchangeable.

Check which standard your bike is running when replacing a chainset or BB, and if in doubt have a professional mechanic do the replacing.

Interesting and informative post I must say, especially for me at this time. As many on the Web have noted, BB technology and the standards have changed and grown over time to the point where someone like me can be a bit confused when trying to decide WTF to do. Anyway, your overall description is greatly appreciated. With that said, I'm wondering if you're familiar with or have ever run across a bicycle with a 76mm bottom bracket shell? My bike, a 20+ year old MTB (a MOTIV Ground Pounder which was purchased at a Price Club before they became Costco) has just that. Seems like all the shells I'm seeing referenced are anything but; 68mm,73mm,79,86.5, etc. Best I could find was mention of an old Raleigh bicycle that had a 76mm shell. Here's what I'm wanting to do. I'd like to install a power meter, either a Quark or Stages unit (not interested in pedal based systems). What I'm having trouble determining is if any BB will work with my unconventional shell width size and allow for me to install a SRAM GXP or Shimano Hallowtech II or other BB along with a new crankset or possibly or even likely an entirely new drive chain. The goal being for me to simply ( yeah right!) install a crankarm-based power meter or a spyder style in the case of a Quark. Any thoughts?